The fidelity of purified DNA polymerases are being assessed in in vitro DNA synthesis assays involving biologically active DNA templates. Both a forward mutational assay, using M13mp2 phage and capable of detecting a wide spectrum of base substitution and frameshift errors, and a reversion assay, using either 0X174 DNA or M13mp2 DNA and capable of detecting relatively specific errors, are being employed. Highly purified DNA polymerases from well characterized procaryotic systems and less well defined eucaryotic organisms all produce base substitution errors in vitro at detectable levels. The exact nature of these errors at the level of DNA sequence is currently underway, as is the role of these enzymes in producing frameshift errors. These studies are intended to provide detailed information on the accuracy of the DNA polymerases themselves, as well as information of the parameters of protein nucleic acid interactions which are important in determining this accuracy.